2.2 Layout Tips for Radiated EMI Reduction in Your Designs

[MUSIC PLAYING] Hello. My name is Denislav Petkov. I'm a systems and applications engineer from the SIMPLE SWITCHER group with Texas Instruments. And today, I want to talk to you about the new LM43603 buck converter EMI and board layout.
Let's go to the screen here and talk about buck converter high di dt loops. The most important thing in switching regulators is looking at the high di dt loops, identifying where those sit on the board, and making those as small as possible. So for a buck converter, the high di dt loop is formed by the input capacitor, the VIN terminal, the high side switch, the low side switch, and the ground terminal.
And keeping this loop as small as possible will make sure that the inductance formed by this loop is also as small as possible. Why is this important? Because higher inductance in the presence of high di dt equals more noise. And that noise is going to show up as high EMI and also can affect the normal operation of the buck converter.
Another topic we should look at is protecting sensitive nodes. The feedback node is a sensitive node and has to be as small as possible. The traces connected to the feedback pin should be short and thin, so they don't pick up any noise.
It's very often that we see this rule violated. The feedback trace on many boards is long and thick. And that is not good, because it affects the performance of the regulator. And it could also affect the EMI.
Shielding is also very important. Here, we have two boards-- two board examples-- one has two layers and the other one has four layers. The component placement is the same. It's the same BOM. Components sit on top. The routing is exactly the same.
The only difference is we have two additional shielding layers in between. And from a test scan that we did here in our 3-meter chamber, we can see that adding additional ground layers in the middle can result in reduced EMI. In this case, we had 5 dB difference below the CISPR 22 Class-B line.
Let's take a look at the LM43603 board layout. We have the board here, but it would be easier to look at the layout on the screen. The LM43603 pinout was designed so that you can have a easy layout and good EMI performance. The whole family of converters has the same pinout. So all of the parts are painted paint-compatible, so your board layout will be compatible with any part from their family.
Now, let's look at the pinout. We said that the high di dt loop in a buck converter is formed by the input capacitor, the high side switch, and the low side switch ground. So in the 43603, the VIN pin and the P ground pins are right next to each other. So that allows you to place the input capacitor as close as possible, making this loop as small as possible, which decreases the inductance in this high di dt loop.
Another important thing to point out is the switches on the opposite side, so that all we have to do is bring switch out and connect it straight to the inductor. You don't have to send the switch node to other layers, because that would generate more noise. Also, the CBOOT pin is right next to the switch, which allows you to place the boot capacitor next to the switch pin, make that also high di dt loops as small as possible.
The feedback node, which is a sensitive node, is all the way down in the corner-- right hand side bottom corner. And feedback is right next to A ground, which provides some shielding and also allows you to place the feedback divider very close to the pin, making the feedback node very small. Since all of these signals are routed on top layer, we can have large unbroken shape here on top, which helps for thermal and also for shielding. And we can have unbroken bottom ground plane, again, for shielding and better thermal performance.
Now, let's go inside our 3-meter EMI chamber and prepare a board for a scan. And now, we're inside TI's 3-meter chamber. This is where we do our preliminary evaluation board testing before we go outside to a 10-meter certified facility. This chamber is lined up with these ferrite tiles to provide the RF insulation.
And this is a very typical setup. We have a 360-degree turntable. The board would sit in the middle here powered up. And here, we have the antenna tower with the antenna attached to it and moving up and down to scan emissions at every height. Let's go and do some testing.
Let's run a scan. Inside the chamber, we have the board powered up. And here, we have a spectrum analyzer, which is getting data from the antenna inside. And on the screen, we're plotting the results from the scan against the CISPR 22 Class-B and Class-A limit lines. And we're almost done here scanning. As you can see, the LM43603 is passing Class-B specification for a 3-meter chamber.
And now, I would like to show you the results we got from a 10-meter certified EMI chamber facility. This is for LM43603, running with 12 volts input, 3.3 volts out, at 3 amps. It is passing Class-B specification. And all the parts in his family were tested and passed the same Class-B test. And for more information, go to simpleswitcher.com. Thank you for watching.